A common problem faced in industry today is the removal of sulfur dioxide, an environmental pollutant formed by the oxidation of sulfur or sulfur-containing substances, from industrial exhaust gases. This pollutant is found as a component in various waste gases such as blast furnace gases, emission gases from certain chemical factories, and flue gases from coal or oil-burning furnaces used in utility plants. For example, in U.S. Pat. No. 3,918,521, to Snavely et al, a steam injection oil recovery system is disclosed wherein the flue gas from an oil-burning steam generator contains sulfur dioxide.
One widely accepted method for attacking this problem of sulfur dioxide removal is the use of wet scrubber systems in which sulfur dioxide-containing gases are intimately contacted with a scrubbing liquor. The scrubbing liquor is selective for sulfur dioxide due to the addition of certain chemicals such as lime, limestone or magnesium oxide.
When the sulfur dioxide-containing gas is contacted with the scrubbing liquor, these noted chemicals react with the sulfur dioxide to form a sulfite-containing reaction product which remains with the liquor, permitting the resulting relatively sulfur dioxide-free exhaust gas to pass on through the process stream. The thus spent scrubbing liquor is then circulated through a regeneration sidestream in which, as the name implies, the sulfur dioxide-selective chemicals are regenerated or replaced; and the thus replenished scrubbing liquor is recirculated into the wet scrubber system. It is due to the oxidation of this sulfite-containing reaction product in the spent scrubbing liquor that many problems arise, as will be illustrated below.
One major type of wet scrubber system for removing sulfur dioxide is known as the "limestone slurry system" in which the scrubbing liquor contains limestone as a sulfur dioxide-selective chemical. Upon contacting the sulfur dioxide-containing gas with the limestone slurry, sulfur dioxide is removed according to the reaction: EQU CaCO.sub.3 +SO.sub.2 +1/2H.sub.2 O.fwdarw.CaSO.sub.3 1/2H.sub.2 O+CO.sub.2. (1)
A major problem experienced with these systems is related to a secondary reaction in which the aqueous oxidation of sulfite to sulfate in the scrubbing liquor occurs. In the pH range of most scrubbers, the reaction is: EQU HSO.sub.3.sup.- +1/2O.sub.2 .fwdarw.SO.sub.4.sup.-- +H.sup.+, or (2) EQU SO.sub.3.sup.-- +1/2O.sub.2 .fwdarw.SO.sub.4.sup.--. (3)
The negative consequences of this oxidation are several-fold as follows:
1. Calcium sulfate, a most tenacious scale, is formed and precipitates or crystallizes on various surfaces throughout the system. In contrast, the formation of calcium sulfite precipitate can be controlled in the scrubber by relying on the usually lower pH and hence favoring the formation of the more soluble bisulfite species: EQU CaSO.sub.3.1/2H.sub.2 O or CaSO.sub.3 (solid)+H.sub.2 SO.sub.3 .fwdarw.Ca.sup.++ +2HSO.sub.3.sup.- +1/2H.sub.2 O. (4)
2. The pH of the scrubbing liquor drops (Equation 2), thus reducing the scrubber efficiency.
3. Examination of scale surfaces in scrubbers occasionally shows calcium sulfate to be the initial depositing species with other constituents forming on the calcium sulfate deposit.
A second major type of wet scrubber system for removing sulfur dioxide from a gas is known as the "double-alkali system" which contains a scrubbing loop and a separate precipitation loop. This system utilizes a sodium based scrubbing loop in which sulfur dioxide is removed from exhaust gas according to: EQU Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O.fwdarw.2NaHSO.sub.3. (5)
If sodium hydroxide is also used, sulfur dioxide is removed according to: EQU NaOH+SO.sub.2 .fwdarw.NaHSO.sub.3. (6)
In the precipitation loop, the spent scrubbing liquor is regenerated by treatment with lime to precipitate the sulfite reaction product according to: EQU 2NaHSO.sub.3 +Ca(OH).sub.2 .fwdarw.CaSO.sub.3 +Na.sub.2 SO.sub.3 +2H.sub.2 O. (7)
There are, indeed, drawbacks in the double-alkali system related to the oxidation of sulfite to sulfate in the scrubbing liquor. As opposed to the bisulfite ion, the sulfate ion is no longer regenerable and is of no further use in the process. This necessitates purging of the sulfate from the scrubbing liquor to avoid calcium sulfate scale. This purging results in the loss of sodium compounds from the scrubbing liquor, which compounds must be replaced at considerable expense by the addition of soda ash, usually to the scrubbing loop.
To inhibit the oxidation of the sulfite species, certain antioxidant agents have been added to the sulfite containing aqueous scrubber medium. For instance, in U.S. patent application Ser. No. 25,304, filed Mar. 30, 1979 (of common assignment herewith), there is disclosed the utilization of polyethyleneamine compounds, to achieve the desired sulfite antioxidation result. Japanese Pat. No. Sho 49-43893, published Apr. 25, 1974, discloses the use of certain aromatic amine sulfite antioxidant additives to effect the desired result.
Despite the advent and use of the above noted sulfite antioxidant additives, one problem that has been encountered in this procedure is that, in those instances wherein the aqueous scrubber medium contains solids, such as fly ash, the effectiveness of the antioxidant additives is reduced, probably due to adsorption of the antioxidant onto the solids particles. Also, the metal surfaces of the scrubber may act to reduce the activity of the antioxidant when used by itself.
Accordingly, it is apparent that there is a need in the art for a method and means for improving the efficacy of sulfite antioxidant agents, especially in those instances wherein solids matter, typically fly ash or the like particulate matter, is contained within the aqueous scrubber medium, or when the metal surfaces of the scrubber tend to reduce the activity of the antioxidant.